The present invention relates to the presentation of continuous-tone images on bi-level display media using ordered dither processing. In particular, the invention is directed to methods and arrangements for generating dither threshold values.
A bi-level display medium, such as a plasma panel, is comprised of a matrix of individual, closely spaced display cells each of which resides in one of two visual states. That is, each display cell is either completely energized (on) or completely de-energized (off). Picture images and other graphic data are readily displayed on a bi-level display medium via selective energization of its cells.
As indicated above, the present invention relates to the presentation of gray-scale, or continuous-tone, images on bi-level display media. Such images are characterized by regions of reflected or emitted light of various intensities. In reproducing the image on a particular display medium, the intensities of these various regions are mapped as closely as possible into correspondingly intense, or bright, regions of the display medium. The greater the intensity of a region of the image, the brighter the corresponding region of the display medium.
Unfortunately, an individual cell of a bi-level display medium can represent only one of two extreme intensities, corresponding to the on and off state of the cell. Thus unlike a cathode-ray tube, for example, a bi-level display medium cannot reproduce gray-scale information at each individual display cell location. However, it is known that the above-described mapping can be achieved in substantial measure by representing the average intensity of a region of the image by a region of the display medium having a corresponding average brightness.
One known method for achieving this average-intensity mapping is referred to as order dither processing. In a so-called "dithered display system", the image to be reproduced is scanned in a matrix of picture elements, each element corresponding to a respective cell of the display medium. The picture element matrix may be conceptualized as being divided into a plurality of n-by-n submatrices, n conventionally being an integer power of 2. Each picture element of each submatrix is assigned a spatially corresponding threshold value from a predetermined n-by-n "dither matrix." As each picture element is scanned, its intensity, conventionally represented by a first multi-bit binary word, is quantized into one of a plurality of intensity levels between predetermined upper and lower quantization limits. At the same time, a dither threshold generator provides a "dither threshold word" representing the dither threshold value assigned to the picture element currently being scanned. If the intensity of a given picture element exceeds the dither threshold value assigned thereto, the corresponding display cell is energized. Otherwise, that cell is maintained de-energized.
Typical dither threshold generators comprise a look-up device, such as a read-only memory, which stores the dither matrix threshold values. As a particular picture element is scanned, the dither threshold value assigned thereto is read out of the memory for comparison to the intensity of that picture element. While such look-up arrangements are generally satisfactory, they are relatively expensive. Moreover, the delay inherent in the look-up process may undesirably limit the speed with which images can be scanned and dither processed.